Biology: Cell Structure
The Animal Cell
A nucleus occurs only in eukaryotic cells. Deoxyribonucleic acid (DNA) and other nucleic acids are stored in the cell’s nucleus. This DNA is the physical carrier of inheritance and, with the exception of some small amounts found in the chloroplast and mitochondrion, all DNA is contained by the nucleus. Ribonucleic acid (RNA) is formed in the nucleus by copying the sequence of the DNA bases (the letters in the genetic code). RNA moves into the cytoplasm for its role in protein synthesis at the ribosome. The nucleolus is an area of the nucleus where parts of ribosomes are formed. The nuclear envelope is a double-membrane structure, perforated by numerous nuclear pores. These pores allow the RNA, ribosomal fragments, and other chemicals to pass out to the cytoplasm. However, DNA does not normally pass through the nuclear pores. Normally, when a cell is dividing, it must disassemble its nuclear envelope. Imagine having a bookshelf too large to fit through your door when moving. This is the general idea of the relationship of the nuclear envelope and DNA.
Ribosomes are the structures in the cytoplasm where protein synthesis occurs. Ribosomes are not membrane-bound, and thus occur in both prokaryotes and eukaryotes.
The endoplasmic reticulum (ER) forms a mesh of interconnected membranes in the cytoplasm that function in protein synthesis, lipid synthesis, and transport.
Rough endoplasmic reticulum (RER) appears rough when seen with the electron microscope. This rough appearance results from numerous ribosomes placed along the edges. RER connects to the nuclear envelope where RNA attaches to the ribosomes.
On the other hand, smooth endoplasmic reticulum (SER) lacks the ribosomes (characteristic of RER) and is thought to be involved in transport and a variety of other functions.
Discovered in the late 1890s, Golgi complexes are flattened stacks of membrane-bound sacs. Not until the late-1900s was the function of the Golgi discovered. The Golgi functions as a packaging plant, modifying vesicles produced by the rough endoplasmic reticulum for export, or for use, within the cell. New cell (and other) membrane materials are assembled in various membrane stacks of the Golgi.
Mitochondria are small, membrane-bounded organelles in almost all eukaryotic cells. They act as the “powerhouse” of the cell by converting the energy locked in sugar molecules into energy temporarily stored in ATP. Mitochondria differ from many other organelles as they are surrounded by two membranes, they contain both their own DNA (termed mDNA), and small, prokaryotic-type ribosomes.
 Try this practice activity to test your knowledge of the Eukaryotic Animal Cell parts.
Now go on to the next part.
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